This invention relates in general to imaging systems, and more particularly, to improved toner image forming and transfer devices and methods.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well-known. The basic xerographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely-divided electroscopic marking material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. The toner image may be transferred to a receiving member such as paper. The transferred toner image may be permanently affixed to a support surface as by heat.
In U.S. Pat. No. 2,297,691, charging is effected, for example, by vigorously rubbing with a soft material such as cotton or silk handkerchief the surface of a photoconductive insulating layer supported on a conductive substrate. The layer is rubbed in the dark so that it will be at its highest insulating value and hence will retain the charge uniformly distributed on its surface. The conductive backing member is normally grounded so that a high potential difference will exist between the charged surface of the photoconductive insulating layer and the conductive substrate. The photoconductive insulating layer is then immediately exposed to a light image or pattern to be reproduced to form a charge pattern on the layer. The exposed layer is thereafter contacted in the dark with toner particles having an electrostatic charge polarity opposite the polarity of the charge on the surface of the photoconductive insulating layer to deposit the toner particles in conformance with the charge pattern on the photoconductive insulating layer. Transfer to paper, metal foil or other sheet material is effected by carefully laying the sheet material on the deposited image and firmly pressing against the surface with a block carrying a felt or sponge rubber pad. This will transfer a part of the power to the surface of the sheet material making the image visible thereon. In order to improve transfer an adhesive is applied to the receiving sheet prior to contact with the deposited toner image. Plain water or other liquids are often satisfactory, especially with paper sheets. Wax parafin or other soft or sticky substances also may be used.
The charging technique disclosed by Carlson achieved voltages which were much too low for practical xerography. Indeed, it is surprising that he was able to demonstrate the phenomena at all, particularly since electrostatic voltages obtained directly by frictional electrification are not only low but unreliable. Electrophotography has progressed dramatically since the filing of U.S. Pat. No. 2,297,691. In most modern electrophotographic copying and duplicating devices of today, charging of the electrophotoconductive insulating layer and transfer of the deposited toner image are effected with high voltage corona charging devices which operate at potentials of from about 2,000 to about 8,000 volts. Expensive and bulky power supply equipment is required to supply these high potentials to the corona charging devices.
Electrostatic recording systems have also been developed which do not require a light pattern exposure system or a photoreceptor. For example, the apparatus and methods described by Frederick A. Schwertz in U.S. Pat. No. 2,978,968 involve electrically forming latent images on a web by positioning the web between a shaped electrode and a reference electrode, forming an electric field between the shaped electrode and the reference electrode to just below the threshold potential at which discharge can begin, and thereafter raising the potential of the reference electrode above the threshold so that discharge takes place from the shaped electrode to deposit electronic or ionic charges on the surface of the web to form an electrostatic latent image. Generally, the potentials applied to the reference electrode above the threshold are at least several hundred volts and usually between about 700 and about 2,000 volts. In another embodiment in U.S. Pat. No. 2,978,968, the web is precharged by exposure to a corona charging electrode energized at a potential of several thousand volts, generally about 6,000 to 10,000 volts. Similar devices and techniques are disclosed by Frederick A. Schwertz in U.S. Pat. Nos. 3,023,731; 3,064,259 and 3,068,481.
Similarly, Robert W. Gundlach describes in U.S. Pat. No. 3,004,860 various known techniques for forming toner images on imaging members which do not require a light pattern exposure system or a photoreceptor. Gundlach also discloses in the same patent a transfer technique in which a toner image on an insulating layer backed by a conductive backing is transferred to a conductive sheet placed on the toner image, the conductive sheet being electrically connected to some point at a potential substantially that of the conductive backing. The point can be the conductive backing itself. However, the toner image and insulating layer must be corona charged prior to contact with the conductive sheet.
While ordinarily capable of producing good quality images, conventional developing apparatus and processes suffer deficiencies in certain areas. Expensive, complex and potentially hazardous corona charging devices in high voltage power supplies are usually necessary to form an electrostatic latent image and/or transfer a toner image. Moreover, conventional electrostatic transfer systems tend to transfer toner particles deposited in background areas of a photoreceptor. Triboelectric charging of a photoreceptor coupled with exposure to a light pattern often produces unreliable results. Reusable photoreceptors and/or the equipment necessary to transport them in conventional electrophotographic copiers and duplicators are expensive to manufacture or occupy valuable machine space. The material and physical property limitations of conventional photoconductors adversely constrain choice of materials and machine design. The optical system in most commercial electrophotographic devices are complex, bulky, fragile, and expensive to manufacture. Thus, there is a continuing need for a better system for forming electrostatic latent images, developing the electrostatic latent image and transferring the resulting toner image.
Accordingly, it is a primary object of the present invention to improve electrostatic imaging systems in which electrostatic latent images, charged toner particles and electrostatic transfer are utilized.